1. Field of the Invention
The present invention relates to porous material suitable for use in bearing rolling elements, and more particularly, to a porous material suitable for use in a bearing rolling element, obtained by a process comprising the steps of mixing degreased bran derived from rice bran with a thermosetting resin before kneading, subjecting a kneaded mixture to a primary firing in an inert gas at a temperature in a range of 700 to 1000xc2x0 C., pulverizing the kneaded mixture obtained after the primary firing into carbonized powders sieved through a screen of 100-mesh, mixing the carbonized powders or the carbonized powders and ceramic powders with a thermosetting resin before kneading, pressure-forming a kneaded mixture thus obtained at a pressure in a range of 20 to 30 MPa, and applying a heat treatment again to a formed kneaded mixture in the inert gas at a temperature in a range of 100 to 1100xc2x0 C.
2. Description of the Related Art
Alloy, and ceramics such as silicon nitride, and so forth have been in widespread use up to now as a material for a rolling element of bearings such as ball bearings, roller bearings, and so forth. The material for the rolling element of such bearings is required to have surface hardness, a small thermal-dimensional-change characteristic, and so forth besides mechanical strength. An alloy-based material naturally comes to have a higher density, causing degradation in bearing characteristics at the time of acceleration due to weight thereof while ceramics are not only susceptible to impact and expensive although low in density but also electrical insulators, thereby requiring means for prevention of electrostatic buildup.
Meanwhile, an attempt to obtain a porous carbonaceous material by utilizing rice bran discharged in quantity of 900, 000 tons a year in Japan and as much as 33 million tons a year throughout the world has been well known by researches carried out by Mr. Kazuo HOKKIRIGAWA, the first inventor of the present invention (refer to xe2x80x9cFunctional Materialxe2x80x9d, May issue, 1997, Vol. 17, No. 5, pp. 24xcx9c28).
Herein are disclosed a carbonaceous material obtained by mixing and kneading degreased bran derived from rice bran with a thermosetting resin, drying a formed kneaded mixture prepared by pressure-forming, and firing the formed kneaded mixture as dried in an minert gas, and a method of producing the same.
With such a method as described above, however, it has been practically difficult to form the formed kneaded mixture with precision because there occurs discrepancy in dimensions by as much as 25% in terms of a contraction ratio of the dimensions of the formed kneaded mixture prepared by the pressure-forming to those of a finished formed product obtained after the firing in the inert gas.
The invention has been developed to solve the problem described above, and it is therefore an object of the invention to provide a porous material suitable for use in a bearing rolling element, having such properties as a small contraction ratio of the dimensions of a formed workpiece formed thereof to those of a finished product, excellent electric conductivity, small thermal strain, insusceptibility to damage, light weight, a long service life, and still ability to retain oil and grease for a long period of time, thereby providing a high-tech eco-material (state-of-the-art material excellent in ecological adaptability) utilizing biomass resources, different from conventional industrial material.
The inventors have been successful in development of a porous material suitable for use in fabricating a high-precision bearing rolling element having excellent properties as material for use in fabricating the bearing rolling element, and a small contraction ratio of the dimensions of a formed workpiece formed thereof to those of a finished product.
The inventors have found out that it is possible to obtain a porous material suitable for use in a bearing rolling element, having such properties as a small contraction ratio of the dimensions of a formed workpiece formed thereof to those of a finished product, excellent hot oil resistance, small thermal-dimensional-change, insusceptibility to damage, light weight, a long service life, and ability to retain oil and grease for a long period of time by making effective use of the degreased bran derived from the rice bran discharged in quantity of 900, 000 tons a year in Japan and as much as 33 million tons a year throughout the world, and has thereby completed the invention.
The inventor of the present invention has conducted intense studies, and found out that a porous material is obtained by a process comprising the steps of mixing the degreased bran derived from the rice bran with a thermosetting resin before kneading, subjecting a kneaded mixture to a primary firing in an inert gas at a temperature in a range of 700 to 1000xc2x0 C., pulverizing the kneaded mixture obtained after the primary firing into carbonized powders sieved through a screen of 100-mesh, mixing the carbonized powders with a thermosetting resin before kneading, pressure-forming a kneaded mixture thus obtained at a pressure in a range of 20 to 30 MPa, and applying a heat treatment again to a formed kneaded mixture in the inert gas at a temperature in a range of 100 to 1100xc2x0 C., and the porous material thus obtained has ideal characteristics as material suitable for use in bearing rolling elements.
More specifically, the porous material described above has the small contraction ratio of the dimensions of a formed workpiece formed thereof to those of a finished product as low as 3% or less, 13 wt % of oil retention characteristic, 4.85xc3x9710xe2x88x923 xcexa9cm of volume resistivity, and density in a range of 1.1 to 1.3 g /cm3, and further, it has been possible to obtain the porous material having Vickers hardness not less than 300xcx9c600, and still friction coefficient in the order of about 0.15 after fired at a high temperature.
Degreased bran derived from rice bran, used in carrying out the present invention, may be either of domestic origin or of foreign origin regardless of the kind of rice.
Further, any thermosetting resin may be used as long as it has thermosetting property, and typically cited as a thermosetting resin are a phenol resin, diaryl phthalate resin, unsaturated polyester resin, epoxy resin, polyimide resin, and triazine resin. In particular, the phenol resin is preferably used.
Furthermore, a thermoplastic resin such as polyamide, and so forth can be used in combination with a thermosetting resin provided that it is done without departing from the spirit and scope of the invention.
As ceramics that can be substituted for a portion of carbonized powders, there are cited Si3N4, ZrO2, Al2O3, SiC, BN, WC, TiC, sialon (solid solution of a Sixe2x80x94Alxe2x80x94Oxe2x80x94N type compound), and so forth.
According to the invention, use can be made of one kind or not less than two kinds of ceramics selected from the group consisting of the ceramics described above. Ceramics not greater than 50 xcexcm in particle size are desirable, ones not greater than 20 xcexcm in particle size is preferable, and further, ones of particle size in a range of 0.3 to 3 xcexcm is more preferably used.
An effect of stabilizing surface hardness is obtained by substituting ceramics not greater than 50 xcexcm in particle size for a portion of the carbonized powders. The carbonized powders can be well mixed with ceramic powders, and a mixing ratio of the former to the latter can be 5xcx9c95:95xcx9c5 by weight.
A mixing ratio of the degreased bran to a thermosetting resin is 50xcx9c90:50xcx9c10 by weight, however, a mixing ratio of 75:25 is preferably used.
For the thermosetting resin used in this case, one in a liquid state, having a relatively small molecular weight, is desirable.
A primary firing is applied at a temperature in a range of 700 to 100xc2x0 C. using normally a rotary kiln for firing time in a range of about 40 to 120 minutes.
A mixing ratio of the carbonized powders after the primary firing to a thermosetting resin is 50xcx9c90:50xcx9c10 by weight, however, a mixing ratio of 75:25 is preferably used.
A kneaded mixture of the carbonized powders after the primary firing and the thermosetting resin is pressure-formed at a pressure in a range of 20 to 30 MPa, and a pressure in a range of 22 to 25 MPa is preferably used. A mold is preferably at a temperature of about 150xc2x0 C.
A heat treatment is applied at a temperature in a range of 100 to 1100xc2x0 C. normally in a well-controlled electric furnace. Firing time is in a range of about 60 to 360 minutes.
As a porous material for use in a bearing rolling element, according to the invention, becomes porous after the heat treatment is applied at a high temperature, Vickers hardness thereof undergoes wide fluctuation, but generally tends to be related to a firing temperature. The porous material subjected to the heat treatment at a temperature in a range of 100 to 400xc2x0 C. has excellent mechanical property.
Conversely, one subjected to the heat treatment at a temperature not lower than 1100xc2x0 C. will have a small Vickers hardness value, and is not suitable as the material for the bearing rolling element.
A warming rate up to the primary firing temperature is required to be relatively moderate up to 500xc2x0 C. In terms of more specific values, the warming rate is in a range of 0.5 to 2xc2x0 C. 1 min, preferably 1xc2x0 C./min.
Further, in lowering the temperature of the kneaded mixture after baked, a relatively moderate cooling rate is required up to 500xc2x0 C. Upon the temperature dropping to 500xc2x0 C. or lower, the kneaded mixture is left to cool by itself. In terms of more specific values, the cooling rate is in a range of 0.5 to 4xc2x0 C./min, preferably in a range of 1 to 2xc2x0 C./min.
Further, for an inert gas, any of helium, argon, neon, and nitrogen gas may be used, however, nitrogen gas is preferably used.
Embodiments of the invention are summed up as follows:
(1) a porous material suitable for use in a bearing rolling element, obtained by a process comprising the steps of mixing degreased bran derived from rice bran with a thermosetting resin before kneading, subjecting a kneaded mixture to a primary firing in an inert gas at a temperature in a range of 700 to 1000xc2x0 C., pulverizing the kneaded mixture obtained after the primary firing into carbonized powders sieved through a screen of 100-mesh, mixing the carbonized powders with a thermosetting resin before kneading, pressure-forming a kneaded mixture thus obtained at a pressure in a range of 20 to 30 MPa, and applying a heat treatment again to a formed kneaded mixture in the inert gas at a temperature in a range of 100 to 1100xc2x0 C.;
(2) the porous material suitable for use in the bearing rolling element, as described under item (1) above, wherein the thermosetting resin is one kind or not less than two kinds selected from the group consisting of a phenol resin, diaryl phthalate resin, unsaturated polyester resin, epoxy resin, polyimide resin, and triazine resin;
(3) the porous material suitable for use in the bearing rolling element, as described under item (1) or (2) above, wherein a mixing ratio of the degreased bran to the thermosetting resin is 50xcx9c90 : 50xcx9c10 by weight;
(4) the porous material suitable for use in the bearing rolling element, as described under any one of items (1) to (3) above, wherein a mixing ratio of the carbonized powders to the thermosetting resin is 50xcx9c90:50xcx9c10 by weight;
(5) the porous material suitable for use in the bearing rolling element, as described under any one of items (1) to (4) above, wherein a particle size of the carbonized powders is in a range of 50 to 250 xcexcm;
(6) the porous material suitable for use in the bearing rolling element, as described under any one of items (1) to (5) above, wherein the thermosetting resin mixed with the degreased bran is in a liquid state, and the thermosetting resin mixed with the carbonized powders is in a solid state;
(7) the porous material suitable for use in the bearing rolling element, as described under any one of items (1) to (6) above, wherein one kind or not less than two kinds of ceramics in powder form, selected from the group consisting of Si3N4, ZrO2, Al2O3, SiC, BN, WC, TiC, and sialon are substituted for a portion of the carbonized powders;
(8) the porous material suitable for use in the bearing rolling element, as described under item (7) above, wherein a mixing ratio of the carbonized powders to the ceramics in powder form is 5xcx9c95:95xcx9c5 by weight;
(9) a process of producing a porous material suitable for use in a bearing rolling element, comprising the steps of mixing degreased bran derived from rice bran with a thermosetting resin before kneading, subjecting a kneaded mixture to a primary firing in an inert gas at a temperature in a range of 700 to 1000xc2x0 C., pulverizing the kneaded mixture obtained after the primary firing into carbonized powders sieved through a screen of 100-mesh, mixing the carbonized powders with a thermosetting resin before kneading, pressure-forming a kneaded mixture thus obtained at a pressure in a range of 20 to 30 MPa, and applying a heat treatment again to a formed kneaded mixture in the inert gas at a temperature in a range of 100 to 1100xc2x0C. before cooling the same at a moderate cooling rate;
(10) the process of producing the porous material suitable for use in the bearing rolling element, as described under item (9) above, wherein the moderate cooling rate is in a range of 1 to 4xc2x0C./min up to 500xc2x0C.
(11) the process of producing the porous material suitable for use in the bearing rolling element, as described under item (9) or (10) above, wherein the primary firing is applied in a rotary kiln;
(12) the process of producing the porous material suitable for use in the bearing rolling element, as described under any one of items (9) to (11) above, wherein the inert gas is nitrogen gas.